The invention relates to an air filter having a depth filter material which contains a nonwoven fabric having a upstream side and a downstream side, the nonwoven fabric being formed of adhesively and/or cohesively bonded fibers.
An air filter of this kind is described in the German Patent 44 27 817 C1.
In filters made of a depth filter material, the particles to be separated are separated predominantly in the filter material. In contrast to this stand surface filters in which the particles are accumulated predominantly at the surface of the filter material as a filter cake.
The German Patent 41 25 250 A1 describes an air filter material which is composed of a laminate made of two layers of nonwoven fabric. The layer facing the downstream side is consolidated by water-jetting, and has fibers of an average fineness below 2.2 dtex. Compared to this, the nonwoven fabric layer facing the upstream side has coarser fibers. Both layers are interconnected by mechanical needling. Such a filter medium has indeed an improved efficacy compared to a purely mechanically needled nonwoven fabric. It is a disadvantage, however, that in laminating by needling, needle holes form also in the layer on downstream side which have a negative effect on the filter efficiency with regard to small particles.
Nonwoven fabrics have proven their worth as filter material for air filters and guarantee excellent filtration properties. However, the known filter materials do not fulfill all wishes with regard to the stability and stiffness of the filter material. On one hand, this is significant in the manufacture of air filters to produce a pleated or folded filter having a correspondingly large filter surface. The stiffness, however, is also of great importance for the function of the filter to prevent an excessive deformation of the filter material during operation. A deformation would cause the filter material to rest against the inside wall of the filter housing, which would result in a very high pressure difference and a short service life of the air filter.
The object of the present invention is to provide an air filter having a particularly stiff filter material.
In the case of an air filter of the type noted above, this objective is achieved in that the fibers have intertwinements which are produced by liquid-jetting from the downstream side prior to the adhesive and/or cohesive bonding. Also, because of the liquid-jetting, the fibers are compacted increasingly from the upstream side toward the downstream side in such a way that differently compacted regions are formed, in which particles of different size can be separated.
The liquid-jetting can particularly be a high-pressure water-jet treatment which is known from the manufacture of nonwoven fabrics. Due to the sharply concentrated high-pressure water jets, which penetrate the nonwoven fabric and strike a base that is permeable to liquids, the fibers are swirled. Because of this, the fibers seized by the liquid jets assume a loop-shaped arrangement. In this context, according to the present invention, the jet energy, which is determined by the liquid mass per unit of time and the liquid pressure, is rated in such a way that the fibers are compacted increasingly from the upstream side toward the downstream side. In this manner, compacted regions are formed, in which particles of different size can be separated.
The liquid-jet treatment from the downstream side carried out in conjunction with the adhesive and/or cohesive fiber bonding results in a nonwoven fabric which has a surprisingly high stiffness and excellent filter properties at the same time. This is surprising because the high-pressure liquid-jetting is used in the manufacture of nonwoven fabrics to produce nonwoven fabrics having a soft, textile handle. A further advantage is that the thicknesses of the filter material of 5 mm and less achieved by the liquid-jet treatment are suitable for an air filter, and can eliminate the need for an additional calendering operation. Moreover, the production cost can be reduced since differently compacted regions can be produced in only one process step also in the case of a single-layer nonwoven fabric.
Advantageously, provision is made for the fibers to have intertwinements which are produced by liquid-jetting from the downstream side prior to the adhesive and/or cohesive bonding. By the additional liquid-jetting from the upstream side, which is preferably carried out using a different jet energy than the liquid-jetting from the downstream side, a particularly firm depth filter material can be obtained.
According to an advantageous embodiment, provision is made for the fibers to have a titer from 0.05 to 50 dtex.
A further improvement of the filter properties is achieved in that the fibers include coarse fibers and fine fibers, the titer of the coarse fibers exceeding the titer of the fine fibers at least by the factor 6. Thus, the air filter can have, for example, fine fibers having a titer of approximately 1 dtex and coarse fibers having a titer of approximately 6 dtex or more.
Particularly small particles can then be separated in the air filter if the fine fibers are at least partially composed of split fibers. Split fibers are relatively coarse multicomponent fibers in usual fiber titers which are capable of being combed, and are easy to process. By cleaving (i.e. splitting) the split fibers, comparatively fine fibers are formed. In this manner, conventional methods can be used to produce also textile fabrics having microfiber structures.
The manufacture of the air filter is particularly simplified by splitting the split fibers using the liquid-jet treatment.
Particularly good dust-storing capacity in the filter material is achieved in that the medium density of the nonwoven fabric increases progressively in the flow direction. Consequently, the filter has a lower fiber density and big filter pores at the upstream side. In this region, the bigger particles to be separated are collected. Toward the downstream side of the air filter, the fiber density increases progressively, the pores formed by the fibers being correspondingly smaller there. Smaller particles to be separated initially pass through the region having a lower fiber density situated on the upstream side, and are then separated in a region having a higher fiber density. In this manner, the air filter can be charged with the particles to be separated over the whole thickness of the filter material. Therefore, long filter endurances and a consistently low pressure loss over the whole filter service life can be achieved.
According to a refinement of the present invention, it is provided for the nonwoven fabric to have at least a first fiber layer facing the upstream side and a second fiber layer facing the downstream side.
The manufacture is particularly simplified in that at least one of the fiber layers is essentially made up of a precompacted nonwoven fabric layer and that at least one other of the fiber layers is substantially formed by a fiber web applied to the nonwoven fabric layer, the fiber web and the nonwoven fabric layer being interconnected by the liquid-jetting.
Particularly good filter properties are achieved by arranging the fiber layer formed by the fiber web at the downstream side of the nonwoven fabric layer.
Particularly small particles can be separated in the filter if the fiber layer formed by the fiber web contains the split fiber.
In a refinement of the present invention provision is made for the fiber layer facing the upstream side to contain a higher proportion of coarse fibers than the fiber layer facing the downstream side. Due to the higher content of coarse fibers in the fiber layer facing the upstream side, bigger pores are formed there. In this context, it can also be provided that the coarse fibers are contained exclusively in the fiber layer facing the upstream side.
A further improvement of the filter properties is achieved by electrostatically charging the fibers. This can be carried out, in particular, by an electret treatment such as a corona treatment.
The cohesive bonding of the fibers can be carried out in a particularly advantageous manner by fusing a part of the fibers together.
The adhesive bonding of the fibers can be carried out in a particularly simple manner by glueing the fibers together using a binding agent. Preferably used are bicomponent binding fibers.
Particularly the production cost of the air filter is reduced by liquid-jetting the filter material over its whole cross-section.
A particularly large filter surface is achieved by pleating the filter material.